How to Read Bolt Head Markings

1. Look at the head: raised or stamped lines and numbers tell you the strength grade.
2. Three radial lines mean SAE Grade 5; six radial lines mean SAE Grade 8; no lines usually means Grade 2 or ungraded.
3. Metric bolts are stamped with property classes — 8.8, 10.9, and 12.9 — beside the manufacturer mark.
4. Stainless bolts show an A2 or A4 steel group plus a strength number, such as A2-70 or A4-80.
5. Letters or symbols identify the manufacturer — useful when tracing a defective batch.
6. Match the bolt grade to the load, and the nut grade to the bolt — never mix grades in a safety-critical joint.

Related Fasteners

• Hex Bolt
• Flange Bolt
• Socket Head Cap Screw
• Structural Bolt